Preliminary experiment:
I carried out preliminary work to see how I would carryout my experiment in my actual plan. It also helped me on deciding suitable apparatus to use for my real experiment. A few potatoes were minced using a mincing machine. 5g of minced potato were then obtained using a balance. I transferred this into a conical flask but with real difficulty. The problem was that I could not get all of the minced potato into the conical flask without leaving at least 0.1g of potato on the balance. I therefore washed the conical flask clean again and wiped the balance. Then I placed the empty conical flask onto the balance and set the balance to zero. I then added minced potato into the conical flask using a spatula until the scale read 5.00g. This was when I placed a bung (with a delivery tube and a small tube attached to it) firmly over the conical flask.
I got two water troughs that were half filled with water and placed it on a large worktop. I then placed the conical flask in the smaller trough. I found that the conical flask floated and realised that this was because there was a lot of water in the trough. I removed water until the conical flask stopped floating. I measured the amount of water in the small trough using a measuring cylinder and noted the measurement. This is so I know how much water I need to place in the trough when I carry out my real experiment. I then placed a beehive shelf in the larger trough and placed a large measuring cylinder of water upside down on it. I then fed the delivery tube underneath the beehive shelf so the opening of the tube was directly under the measuring cylinder ready to collect any oxygen produced.
I measured the temperature of the water in the small trough using a thermometer. I noted down the temperature to make sure that the temperature is constant in every repeat. I got a syringe and measured 20ml of hydrogen peroxide and also got a stop clock. I used a 20ml syringe because I was using 20ml of hydrogen peroxide and I think it is convenient to use a 20ml syringe. The mouth of the syringe was pushed into the small tube on the bung. As soon as the stopwatch was started I pushed the syringe so that the hydrogen peroxide solution could enter the conical flask. I timed the reaction for two minutes and observed closely. Here are the results I obtained:
From these results I have decided to time for only 30 seconds. This is because the result values are very high and there will be no way of measuring so much gas using a single measuring cylinder. However I did consider using two or three but thought that changing the cylinder will take time and I would lose gas whilst changing the cylinders. I have also decided that my initial readings will be 2.5g and I will increases the mass as 2.5g, 5g, 7.5, 10g, 12.5g, etc. I have decided this because I will be able to achieve more results and it will be able to see a trend on the graph. My final reading will be 15g. I have decided to repeat the experiment 3 times each so I get a more accurate set of results and so I can check the validity of my results and to minimise human errors. I will also be able to obtain an average from these results.
Plan
Apparatus:
∙Potatoes
∙Hydrogen Peroxide∙100ml Measuring cylinder∙Conical flask∙Beaker
∙Beehive shelf∙Bung (with a small glass tube and delivery tube attached to it.) ∙Thermometer (100°C)∙Water∙Mincing machine ∙1 small water bath
∙1 large water trough
Diagram:
Method:
The apparatus will be set up as shown in the diagram. A few potatoes will be minced using a mincing machine for about 30seconds. 127ml of water will be measured using a measuring cylinder and transferred into a small water trough. Then a large water trough will be filled just above halfway and a beehive shelf will be placed in the middle of the large trough. A measuring cylinder then will be filled with water and placed upside down over the beehive shelf making sure that there are no air bubbles in the measuring cylinder. The conical flask will be placed on the balance and the scale will be reset to zero. Minced potato will be added to the conical flask until the scale reads 2.5g. The bung with the small glass tube and delivery tube attached will be tightly secured into the conical flask assuring that no gas will escape. The conical flask will then be placed into the small water trough. The temperature of the water in the small trough will be measured and the temperature will be kept constant throughout the experiment. If the water turns cold then cold water will be added and vice versa. 20ml of hydrogen peroxide will be measured using a syringe and will be slotted into the small glass tube in the bung. The stopwatch will be started immediately when the hydrogen peroxide is injected into the conical flask. After 30 seconds the amount of gas collected in the measuring cylinder will be entered into my results table. I will make sure that I read the measuring cylinder at eye level and at the bottom of the meniscus. The conical flask will be washed out thoroughly and the measuring cylinder will be emptied. Then I will repeat the experiment again. I will carry out 3 repeats on each mass and I will enter the results in my results table. The experiment will be repeated three times so that any inaccurate data can be seen. The reason the experiment is going to be repeated so may times is because each time it is done it will be hard to control so that it is in exactly the same conditions each time. Things that will be hard to keep exactly the same like the amount of hydrogen peroxide used will have to be measured very accurately with measuring instruments. Another reason why it is repeated is because there are other factors like a change in temperature that might affect the results of the experiment and they will not be able to be controlled with a lot of accuracy.
Hydrogen peroxide is corrosive and irritant therefore, to ensure this experiment is carried out safely; I plan to wear goggles to protect my eyes from the hydrogen peroxide. Care also needs to be taken to prevent hydrogen peroxide getting on clothing. Wearing an overall will prevent this. Also I will be using a lot of glass in this experiment, like the conical flask, delivery tube etc. Therefore I will need to handle these objects with care so that I do not cause any breakage or harm.
To ensure my results are reliable I will keep the following constant in my experiment:
- The volume of hydrogen peroxide - by measuring out 20ml using a syringe accurately.
- The concentration of hydrogen peroxide - by making sure that I do not dilute the hydrogen peroxide with water.
- The pH - by keeping the same type of potato in throughout the experiment.
- The surface area of the potato - by mincing the potato for the same amount of time, i.e. 30seconds.
- The temperature of the water bath - by measuring the temperature using a thermometer and making sure that the temperature does not change. If it does then I will add hot or cold water to make the temperature same again.
- The time I will be timing for in each experiment – by timing each reaction for 30seconds using a stop clock.
- The apparatus I will be using throughout the experiment – by using the same apparatus and just washing them to reuse them.
The experiment will be made fair by using the same volume of hydrogen peroxide in the experiment every time. I will also use the same type of potatoes. I will also repeat the experiment three times so that any abnormal results will be seen. I will use a measuring cylinder instead of counting the bubbles of oxygen released to measure the amount of oxygen produced. I will read the measurement on the measuring cylinder at eye level so that I can read where the minuscule is.
SKILL AREA O: OBTAINING EVIDENCE
Discussion:
Make sure that both the hydrogen peroxide and the piece of potato are the same temperature as the water before adding them together to make the test fairer and more reliable
SKILL AREA A: ANALYSING AND CONSIDERING EVIDENCE
The graph to show the volume of oxygen produced over an increase in concentration of catalase shows that, in general, as the concentration of catalase increases, the volume of oxygen also increases. This was due to the increasing number of molecules of catalase enzyme breaking down the molecules of hydrogen peroxide substrate. When the concentration of catalase is increased, there are more particles per unit volume. Therefore, the frequency of collisions increases, and the rate of reaction increases. It did this because catalase is breaking enzyme. Being a breaking enzyme, each catalase molecule acquired a molecule of hydrogen peroxide substrate on its active site. Here, the enzyme divided up the substrate into much smaller parts.
I have drawn a curve as the line of best fit as the points on the graph do not show a linear relationship. At the beginning of the graph the gradient is steeper, this is when the reaction is faster in the middle of the graph the gradient of the line is less steep because the reaction is slower. The graph then levels off and the gradient of the line is zero. This is when the reaction is complete because all of the hydrogen peroxide has been reacted and is decomposed fully.
I have drawn construction lines on my graph when the mass of potato is 5g and 10g. When the mass of potato is 5g, 19.75cm³ of oxygen is produced. When the mass of potato is 10g, 25.25cm³ of oxygen is produced. This shows us that when the mass increases my 5g the volume of oxygen produced also increases by roughly 5cm³.
This is because more catalase is being added, increasing more active sites for the reaction between catalase and hydrogen peroxide to take place.
Overall the results recorded and the graphs subsequently produced do mostly agree with the prediction I made. The more catalase used did speed up the reaction rate between the catalase and hydrogen peroxide. This was due to the higher concentration of catalase reacting with the same volume of hydrogen peroxide. Also, as I predicted as the surface area increased, the reaction rate did increase. The reason for this was that the greater surface area allowed more active sites on the catalase substrate so more hydrogen peroxide could be reacted with.
More enzymes mean more substrate to be reacted with.
SKILL AREA E: EVALUATING
Evaluation
I carried out the method I produced following all the fair testing points I made as well as all the safety precautions I listed. I used all the equipment I listed.
The method was carried out well and accurately. The majority of results agreed with my original prediction. The results all seemed to follow the same pattern. Some points were away from the graph due to experimental errors. Another possibility is that I may have timed the experiments more than 30 seconds as looking from the stopwatch to the measuring cylinder could have wasted a couple of seconds. Other factors that may have affected the experiment are; change in water temperature, difference in hydrogen peroxide concentration, accidental release of oxygen bubbles or samples of either substance containing significant quantities of impurities. I could have ensured more accurate results by using more accurate equipment, such as the scales that could only measure correctly to two decimal places or the measuring cylinder that would have been more accurate had been a lot thinner.
On the graph I have drawn a curve of best fit as I thought that this was better joining the point than drawing a line of best fit. There were some points on the graph that do not touch the curve. This may be due to experimental errors like not checking that the environment does not affect the reaction rate. For example I did not check the temperature of hydrogen peroxide and the temperature of the potato. They were room temperature. As I had carried out experiment into more that one lesson the room temperature must have changed, created experimental error. If I had extended my experiment and carried on obtaining evidence then I assume, looking at my graph, that the volume of oxygen would be constant even though the mass of potato was increased. This is because all of the active sites have been met and all of the hydrogen peroxide molecules have reacted.
There was sufficient evidence from the testing I carried out, to support my conclusion because the results I obtained were enough to agree with my original prediction. I could have continued testing different dependant variables, which would have proved or disproved my prediction correct. Further work to reach a better conclusion may be to research into more scientific theories and carry out experiments repeatedly until an accurate set of averages is achieved. If all the independent variables were experimented with as dependant variables then a nearly perfect conclusion could be drawn.
My results are reliable as all of the results I obtained in my repeats are similar to each other giving me a reliable average. The graph also agrees with this as my points show a trend and I was able to show a line of best fit on it. I have no anomalous results as all the points are close to the line I have drawn on the graph not all of the points are on the line showing experimental errors.
Many variations of this experiment could back up the evidence I already have. The independent variables could be changed. For instance; the water temperature could be varied, the hydrogen peroxide concentration could be altered or the surface area could be varied more by using differently shaped potato samples. Further testing, using more catalase or allowing the reaction to continue for longer would show if the reaction would level off. Obviously, I could improve upon the same experiment that I have already done by repeating it five times and therefore getting a better average. I could have taken more readings so that my graph had more points and the trend could be seen more clearly. I think that my method for finding how the concentration of catalase affects the rate of the reaction between catalase and hydrogen peroxide was the best possible one under school laboratory conditions.
I could improve my experiment by using a gas syringe instead of an upside down measuring cylinder filled with water. As the gas syringe is specially made for collecting gas, this apparatus will be more reliable for collecting oxygen for the investigation. If I were working with a partner, this would also make a difference in the results as one person could be observing the experiment whilst the other was looking at the time. Not only would the results be more reliable, but also the investigation would be carried out faster allowing more time for more readings to be taken down. In between the person who was mincing the potato was changed in my investigation and so the potato could be minced differently at times. Although this did not make a huge difference to my results, I think that if the same person does it throughout the experiment then the results plotted on the graph me be mre closer to the line or even on the line.